The general principles of functional electrical stimulation (FES) are rooted in the physiological process of nerve and muscle excitation. These excitations are a result of action potentials (APs) that occur in the body at the neuronal level. APs are the messenger signals for the nervous system. They occur in nervous system tissue in response to stimuli, which can be natural or artificial. In the case of FES, these stimuli are charge pulses. Depending on the amplitude, duration and frequency of these stimuli they can cause excitation in different tissues. FES therapies use these excitation pulses to treat patients with impairments in different areas of the body. Owing to the complexity of generating APs, the artificial electrical stimulation pulses which can generate these APs may require particular pulse types and stimulation schemes for FES applications.
All body cells display a membrane potential, which is a separation of positive and negative charges across the membrane. This potential is related to the uneven distribution of potassium ions (K+ ions), sodium ions (Na+ ions) and large intracellular protein anions between the intracellular and extracellular fluid and to the differential permeability of the plasma membrane to these ions.
Two types of cells, muscle cells and nerve cells, have developed specialized use for this membrane potential. Nerve and muscle are excitable tissues that by changing their resting potential are able to produce electrical signals—or action potentials (APs)—to communicate. FES uses artificial stimuli in the form of electrical pulses to elicit excitation in different tissues.
Neuromuscular electrical stimulation (NMES) is one of the useful therapeutic methods to improve motor function. Studies examining the use of NMES have demonstrated improvements in joint range of motion, force and torque production, magnitude of electromyographic (EMG) muscular activity, and reduction of muscle tone. While the above studies used NMES for single-segment exercise and muscle strengthening, more recently, some studies have been focused on the effect of electrical stimulation on improving and/or restoring voluntary functions such as grasping, walking, reaching, breathing, swallowing and so on in severely disabled individuals. Functional electrical stimulation (FES) is a device-mediated therapy that integrates electrical stimulation of sensory-motor systems and repetitive functional movement of the paretic limb or a body part or a body function in patients with different forms of neuromuscular disorder, such as stroke, spinal cord injury, multiple sclerosis, cerebral palsy, and traumatic brain injury, to name a few.
Known FES devices, although useful, have had limited success at reaching their full potential. For example, previous devices have not been able to ensure charge balance over time because of partial control over temporal characteristics and amplitude. They also provide a limited number of pulses and require complicated and costly adjustments for use in different FES applications. The inflexibility of these designs, in some cases, translates to underutilization of FES therapy.
Nonetheless, various functional electrical stimulators have been used over time to improve the lives of patients with various neurological and musculoskeletal disorders and muscular atrophies as well as in therapy for sport injuries. Known FES devices provide electrical pulses activating a single or a group of muscles, to create a movement (neuroprsthetic applications) and/or build up the muscle mass (neuromuscular stimulation applications). FES devices have also been used in treating bladder problems, easing the symptoms of Parkinson's disease and numerous other applications. Generally, for each application a specific FES system is used.
In emerging sophisticated applications, such as FES therapy, brain machine interface controlled neuroprostheses for grasping and close-loop controlled neuroprotheses for sitting and standing balance, the FES systems would generally have to provide a much wider range and variety of pulses compared to conventional application-specific systems. For example, sophisticated FES applications may generally require the output power stage to produce pulses for which amplitudes, durations, shapes and/or frequencies can be changed in real-time from one pulse to the other, for example. FIG. 1 shows common classifications of pulse shapes that can be used in FES applications. Systems allowing for the seamless transition from one pulse shape to another could allow for greater treatment flexibility. Furthermore, systems demonstrating greater power efficiency and having a relatively small volume may promote greater sustainability as battery powered portable systems, for example.
Known stimulators typically produce either voltage or current regulated electrical pulses. In recent years, the latter have been more widely accepted, because the current regulated pulses generally deliver the same amount of charge to the tissue regardless of tissue resistance. However, the current mode solutions suffer from potential problems related to partially detached electrodes that can suddenly increase the resistance path and, consequently, result in an overly large voltage. The excessive voltage may cause discomfort and burns in the patient with reduced or loss of sensation. On the other hand, an equivalent scenario can be envisioned with voltage mode controlled stimulators. A sudden reduction in the tissue resistance, due to the voltage breakdown of the tissue, may cause an abrupt increase in the stimulation current. Hence, it could be desirable to regulate both voltage and current. From the practical implementation point of view, most current source solutions have a disadvantage of operating output transistors in linear mode, which results in undesirable heat dissipation. Consequently, the battery life is significantly reduced and the overall size of the power stage is often significantly increased due to additional cooling requirements.
Another parameter of interest in these applications is the rise time, i.e. the slew rate of the electrical pulses, which, in general, should be as fast as possible. Namely, the relevance of providing a fast rise time in these pulses stems from the physiology of excitable tissues, namely nerve and muscle cells, and the generation of action potentials. These tissues have ion pumps that work against the delivered charge of an electrical pulse to maintain the nominal potential difference on the cell membrane. Pulses with a higher slew rate may give less time to the ion pumps to compensate for the delivered charge, allowing stimulation with lower amplitude signal. The advantages of stimulating with lower amplitude pulses may include more comfortable (i.e. less painful) therapy and a longer battery life of the device, for example.
Another parameter of interest in these applications, particularly where bipolar pulses (see FIG. 1) are used, is that the net electric charge brought by each pulse be as close to zero as possible, which parameter generally applies in the application of symmetric and asymmetric bipolar pulses. This feature is generally considered relevant in preventing or at least reducing charge accumulation in the tissue, which may cause a galvanic process that may lead to tissue breakdown, for example. To address this problem, auxiliary discharging circuits are frequently used in known devices, where after each bipolar pulse, the accumulated charge due to unbalanced operation is released on a resistor, for example. While this solution may result in zero accumulation, it generally increases heat dissipation and reduces maximum pulse frequency, i.e., due to the extra time needed for the discharge.
Based on the above and other drawbacks, most of the conventional application-specific FES systems cannot be directly used in emerging FES applications. For example, they generally cannot offer a sufficiently wide range and variety of pulses for such applications, they are generally unsuitable for sustainable battery-operated solutions given overly large power consumption significantly limiting their operational time, and/or are generally unable to simultaneously provide signals for multiple channels, for example, which may be of particular relevance in systems such as neuroprostheses for standing and walking. These limitations mostly come from the output power stage that, in the current designs, operates as a lossy linear mode current source causing heat dissipation. Another drawback to predominantly used current source based devices is their signal slew rate, which is significantly smaller than that of the voltage mode systems, thus further contributing to power losses and resulting in a less comfortable therapy.
Accordingly, there is a need for a functional electrical stimulation (FES) device and system, that overcome some of the drawbacks of known technologies, or at least, that provide the public with a useful alternative.
The above background information is provided to reveal information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.